Plant or vegetable protein products have been consumed for centuries in
Asia and Middle Eastern countries. It was not until the mid-20th century
that the western world recognized their food value, especially
that of soybeans. With the growing interest in reducing the
consumption of animal products for health and economic reasons,
plant proteins are making up a higher proportion of the
human diet in recent years. Soybeans (18-20% oil) are the major
source of edible oil in the United States and by far the most
important source of vegetable protein ingredients. However, after the oil
is removed, the remaining soybean meal is used mostly as animal
feed, with perhaps less than 10% being used directly for human
food.

The problem with soybeans is that it also contains some
undesirable components that must be removed or reduced to
increase the usefulness and functionality of the soybean protein
(almost all sources of plant protein share this problem). For
example, soybeans contain oligosaccharides that have been
implicated with gastrointestinal stress. Lipid-lipoxygenase
interactions must be avoided to prevent painty off-flavors from
developing. Phytic acid forms insoluble chelates with minerals
and can form complexes with proteins that reduce bioavailability
of the minerals and proteins. Trypsin inhibitors are
proteinaceous compounds that affect the efficiency of protein
digestion.

Traditional processing techniques for producing soy protein
concentrates and isolates partially overcome these problems.
These methods involve extraction, heat treatment and
centrifugation to separate the protein and fat from the other
components. These conventional methods are time-consuming, they
sometimes result in products with poor functional properties,
and can generate a whey-like waste stream which contains a
significant portion of the proteinaceous compounds of the
starting material.

We have developed alternate
processes for purifying vegetable proteins and removing many objecionable flavor compounds using membrane
technology. Since the undesirable oligosaccharides, phytic acid
and some of the trypsin inhibitors are smaller in molecular size
than proteins and fat components, it should be possible, by
careful selection of the membrane and operating parameters, to
selectively remove these undesirable components and produce a
purified protein isolate or lipid-protein concentrate (depending
on the starting material) with superior functional properties.

Full-fat soy protein concentrates (soymilk)
The process shown in Figure 1 can be used to produce a purified
protein-fat concentrate or a "soymilk" devoid of
oligosaccharides and with lower trypsin inhibitor, reduced off-flavors
and low in phytic acid. Whole soybeans are soaked and then blanched to
prevent lipoxygenase-induced off-flavors during grinding. The
first separation step (filtration or centrifugation) serves to
remove insoluble carbohydrate and fiber and ensure the particle
size is appropriate to the membrane being used. Ultrafiltration
of these water extracts with 20,000 -500,000 MWCO membranes have
been reported. The composition of the full-fat products obtained
with a 50,000 MWCO membrane is shown in Table 1. Using higher
MWCO membranes did not change the final product composition
much, but resulted in much higher flux. Reducing the pH of the
extracts to pH 2 improved flux but resulted in off-flavors in
the product due to the hydrolysis of the oligosaccharides.

Table 1. Composition of soy products (% dry
basis) produced by ultrafiltration of water extracts of soybeans

Soy protein concentrates and isolates
As shown in Figure 2, the raw material is defatted soy flour.
After extracting under optimum conditions of temperature,
meal-to-water ratio and pH, the extract can be directly
ultrafiltered with large diameter tubular membranes to remove
the oligosaccharides. The retentate's final composition will
approximate a soy protein concentrate (70% protein, dry basis).
To produce isolates (90% protein), the fiber and insoluble
carbohydrate is removed by centrifugation or filtration prior to
UF. The underflow from the centrifuge (or the filter cake) can
be re-extracted if necessary. Ultrafiltration at pH 9 increases
flux 40% compared to pH 5.2. A sequence of UF, continuous
diafiltration and UF appears to be optimum.

Removal of
oligosaccharides follows theoretical predictions and is
relatively unaffected by operating parameters. However, removal
of phytic acid is affected by pH. Phytic acid has a molecular
weight of 880 and should be easily removed by UF. However, it is
a negatively charged compound and can bind directly with soy
protein by charge interactions at low pH, or through a divalent
cation bridge at high pH. Thus the pH, nature and concentration
of other charged compounds present during ultrafiltration has a
profound effect on the removal of phytic acid. UF at pH 8.5 or
pH 3 results in strong binding of phytate to the protein, and
thus no removal will occur. Charge interactions are minimized
near the isoelectric point of the protein (pH 5). Divalent
cations become more soluble and binding of phytate to the
protein decreases, thus allowing it to be removed. Adding EDTA
to the extract at high pH or excess calcium at low pH also
effectively reduces the binding.

The manufacture
of soy products by ultrafiltration usually results in higher
yields because of the inclusion of the whey proteins that are
normally lost in conventional manufacturing methods. These whey
proteins could also be contributing to the superior functional
properties of the UF soy products, in addition to the benefits
of the nonthermal and nonchemical nature of the UF process.

Our laboratory has done work on several plant proteins, such as corn (maize),
sunflower(Helianthus annus) and dry beans
(Phaseolus vulgaris L.). More details are available in our
publications list.